1. Field of Invention
The present invention relates generally to insulation attachment studs and methods of attaching insulation to structures. More particularly, the invention relates to a stud and attachment assembly for securing individual studs to a composite material substrate so that thermal or acoustic insulating materials may be attached to the substrate.
2. Brief Description of Related Art
The Navy has studied the use of organic matrix composite materials, such as glass reinforced plastic (GRP), as structural components. Organic matrix composite materials exhibit high structural strength, resistance to the marine environment, and ease of manufacture. Thus it has been proposed that vinyl-ester and glass composites be employed as the structural material for an all composite deckhouse module.
However, the principal hindrances to the use of organic matrix composite materials as structural components are that exposure to increased temperatures reduce their strength and structural integrity and that close proximity exposure to high intensity heat sources can cause them to burn. In fact, Navy studies indicate that such composites are vulnerable to loss of strength at relatively low temperatures. Consequently, passive fire protection is required to protect organic composites from fire-related heat effects.
The Navy presently employs passive fire protection systems to protect metallic structural components on surface ships from fire. Passive fire protection systems are based on an insulation layer composed of mineral wool blankets with an outer sheet of fibrous glass cloth. Studs are welded, using a spot welding gun, to the metallic components and the insulation batts are impaled on the studs and held in place by steel retaining caps. However, there is no method available to weld metallic studs to organic composite materials. Thus there is a need for a means and method of attaching layers of insulation to organic matrix composite material structures in order to protect such structures from the detrimental effects of fire or other heat sources.
The Navy has studied a number of options for attaching fire protective thermal insulation to composite structures. Among the options considered are: embedding studs having enlarged bases within the composite; embedding nuts within the composite and inserting threaded studs into the nuts; embedding metal plates within the composite and using sheet metal screws to attach studs; drilling holes in the composite, installing threaded inserts in the holes and inserting threaded studs into the inserts; drilling through the composite and using through bolts to attach the studs; and either adhesively bonding the insulation layer directly to the composite or adhesively bonding a suitable baseplate having studs welded or otherwise bonded thereto directly to the composite.
However, all these proposals involve potential problems. Embedded systems would have to be installed during lay up of the composite material panels and, thus, would require that the panels be made with prior knowledge of the pattern and placement of insulation studs. Stud location is dependent on placement of insulation batts which is dependent on final placement of bulkheads and ancillary equipment, location of cable runs, and other factors involved in building a ship. Because proper stud location is very important to properly holding the insulation material in place, insulation batts have in the past been installed on an ad hoc basis. Changes in the ship's configuration or addition of bulkhead mounted equipment may require changing the location of insulation batts, thus, requiring alternative means of attaching the insulation in the field. Additionally, the composite panels must have sufficient thickness to securely hold the embedded elements. Currently, the Navy uses a vacuum bag process to produce composite panels. Including embedded studs in the manufacture process may cause holes in the bags and, thus, make manufacture of composite panels more difficult. Furthermore, the embedded elements and inserted studs must not over-penetrate and cause delamination of the composite layers.
Through bolts have a tendency to compress the composite material between the securing nuts and to increase the ship's radar cross-section due to the presence of bolt heads on the exterior of the deckhouse. Through holes may cause water leakage through exterior walls and bulkheads and could weaken the composite structure. Furthermore, through bolts can not be used in areas where both sides of the composite are not accessible.
Adhesive bonding requires an adhesive that is compatible with the composite material at normal and high temperatures and which will last the projected 30 to 50 year life of the ship. Presently, the Navy's passive fire protection insulation is based on a mineral wool blanket with an outer sheet of fibrous glass cloth. Directly adhering the insulation to the structure would require that the insulation be redesigned to accept adhesive on its inner surface. A compatible design would include a protective outer layer of fiberglass scrim, a mineral wool insulation, an additional inner layer of fiberglass scrim, and a quilt construction using high temperature threads to sew through the two fiberglass scrims. Adhesively bonding a base plate with attached stud directly to the structure would not require redesigning the insulation. However, recent tests indicate that adhesives may degenerate under heat load, especially near the limits of operation of the insulation.
To increase the high temperature bond strength of a system employing an adhesively bonded base plate with attached stud, it has been recommended to secure the base plate to the composite structure using two to four sheet metal screws in addition to an adhesive. This method would involve four operations: (1) laying out pilot holes for the screws; (2) drilling pilot holes; (3) applying glue to the base plate; and (4) positioning the base plate while driving the screws in place. While the non-embedded systems have the advantage of not necessitating a prior knowledge of required stud position, they are all very time and labor intensive since approximately three studs per square foot of surface area are required to hold the insulation in place.